These messages appears because GTK is not fully installed, to solve that just do : sudo apt-get install gtk2-engines-pixbuf

To explain more : when you start Bouml without a license I immediately show a file dialog to save the license request, by default the file dialog used by Qt is the native file dialog of Ubuntu and this one uses GTK.

I installed Ubuntu 11.10 64b under a VM (I don't know why but it wasn't possible to install it directly out of a VM on my PC), I have the same warnings, after the installation of gtk2-engines-pixbuf the warning stops but I have a crash like you. The viewer doesn't crash but strangely when it starts it shows the open file dialog like if we ask for to load a project.

The QT version is the 4.7.4, you can see that starting the viewer (boumlViewer) and choosing About Qt in the menu Help

After analyze the problem comes because Qt strangely and wrongly send events associated to the dynamic menus when they are configured, even it seems I found a way to ignore these wrong events I really don't like.

testing on all versions of Ubuntu I have :

Ubuntu 10.04 64b Qt 4.6.2 : OK

Ubuntu 10.04 32b Qt 4.6.2 : OK

Ubuntu 10.10 64b Qt 4.7.0 : OK

Ubuntu 10.10 32b Qt 4.7.0 : OK

Ubuntu 11.04 32b Qt 4.7.0 : OK

Ubuntu 11.10 64b Qt 4.7.4 : KO

Ubuntu 11.10 32b Qt 4.7.4 : KO

and :

Windows 7 with QT 4.8.0 (the version I distribute) : OK

Windows 7 with Qt 4.7.4 : OK

CentOS 5 with Qt 4.2.1 : OK

Debian Wheezy (testing) with Qt 4.7.4 : OK

visibly the problem concerns only Ubuntu 11.10 Qt 4.7.4, it is surprising to not have the same behavior between Ubuntu and Debian with the same version of Qt